Vehicle operating efficiency may be greatly affected by fuel economy performance. One contributor to reduced fuel economy is a high minimum engine idle speed, because all fuel that is consumed at idle does not contribute to vehicle movement and thus lowers the vehicle operating efficiency. The biggest restriction to reducing engine idle speeds and consequently reducing this wasted fuel usage is the need to power engine accessories and quickly compensate for changes in these accessory loads. One such load is the power steering system.
Most automobiles are equipped with a hydraulic power steering system. This system mounts a hydraulic pump on the engine accessory drive. As the steering wheel is moved, the steering gear uses hydraulic pressure from the pump to assist with turning the vehicle wheels. Suspension design and power steering gear design can result in very high and difficult to predict hydraulic loads which cascade as engine loads. This happens frequently at idle, and can result in large fluctuations in engine speed. One approach to compensate for fluctuations in engine load includes setting the engine idle speed higher than might otherwise be necessary in order to mitigate the fluctuations. In another approach, a power steering torque requirement used to control engine idle speed is estimated based on a steering wheel angle sensor signal. An example of this approach is disclosed in U.S. Pat. No. 5,947,084.
However, the inventors herein have recognized various issues with the above approach. For example, estimating power steering torque load based directly on a signal from the steering wheel angle sensor may result in inaccuracies in torque estimation. In particular, a steering wheel sensor may only generate a signal that indicates an angle of the steering wheel that is relative to a steering wheel position at vehicle startup. The steering wheel angle sensor signal is not relative to a center or end-of-travel position of the steering wheel. Thus, the power steering load estimation of the above described approach may not identify particular absolute steering wheel angular positions that cause increases in engine load. Such estimations may result in less accurate engine idle speed control that utilizes a higher minimum idle speed that leads to increased fuel consumption.
The above issues may be addressed by a method for controlling engine output of an internal combustion engine of a vehicle having a hydraulic power steering system during an idle condition to compensate for variations in engine load due to operation of the power steering system. One embodiment of the method may include, during an idle condition where an engine speed is set to an idle speed, adjusting engine output based on a learned absolute steering wheel angle to vary the engine speed from the idle speed to compensate for changes in engine load caused by operation of the hydraulic power steering system. The learned absolute steering wheel angle may be based on a steering wheel angle relative to a steering wheel position at vehicle startup and operating conditions from previous vehicle operation before the vehicle startup.
By learning an absolute steering wheel angle that is defined relative to a center position of the steering wheel, regions of steering wheel angle defined relative to the center position where power steering operations contribute to increases in engine load may be accurately identified. The accurate identification of such regions may allow for more accurate adjustment of engine operation to compensate for the variations in engine load. Accordingly, the minimum engine idle speed may be reduced. In this way, fuel economy may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.